Hi. Localization issues has been discussed here, try a search : i18n and localization.
I think for you the problem is the way Nukes recognizes the language to use. By default it's the language given in your browser. Unfortunately, ES should be added to the "known list" in coremodule.java. For my own needs I had to force FR use. If you use ONLY ES, I guess you should do the same.